Increasingly stringent pollution standards have resulted in the standard practice of using a double seal arrangement in the space between a floating roof and the inner walls of a fuel storage tank. There exist many types of double seal arrangements for floating roofs. U.S. Pat. No. 4,308,968 (1982) to Thiltgen et al. discloses a primary vapor barrier bolted between the floating roof and a shoe which rides on the wall of the tank. A secondary seal comprises a flexible support arrangement mounted on the floating roof. The flexible support has flexible wipers pushing against the inner wall of the tank. The secondary seal projects about 24 inches above the primary seal. The disadvantage of this arrangement is that the storage tank loses 24 inches of its storage capacity.
FIG. 1 shows another commonly used arrangement. A floating roof 1 floats atop the liquid 10. A shoe 5 slides along the inner wall 4 of tank 2. The shoe 5 is pushed against the inner wall 4 by a pusher spring 6 which is mounted against the outer rim plate 7. Brace 8 helps support the shoe support 9. The shoe support 9 also pushes shoe 5 against inner wall 4. The volatile liquid 10 evaporates vapors 11 which are trapped by a primary seal comprising a shoe 5 and a primary vapor barrier 12. Primary vapors 11 create a saturated vapor space. Primary vapor barrier 12 is mounted between shoe 5 and outer rim plate 7. A compression plate 14 supports a secondary seal 13 against the inner wall 4. This arrangement satisfactorily seals in the secondary vapors 110.
However, the compression plate 14 rises a height h2 above the primary vapor barrier 12. The height from the liquid surface 16 to the tank top 15 is h1. The height h2 is lost for liquid storage purposes.
The present invention eliminates the lost storage height h2 while providing the same double seal protection shown in FIG. 1. It would not be effective to merely put a secondary vapor barrier parallel to the primary vapor barrier 12 and mounted to the shoe 5 and outer rim plate 7. Due to the irregular shape of the inner wall 4 there is often a vapor leak between shoe 5 and inner wall 4. Therefore, merely stringing another vapor barrier from shoe 5 to outer rim plate 7 would not capture any fumes from a vapor leak between inner wall 4 and shoe 5.
The present invention utilizes a double shoe arrangement with a primary and secondary vapor barrier extending to the outer rim plate.